Tantalum electrolytic capacitors are widely used in electronic equipment because they can be easily miniaturized. An anode for a tantalum electrolytic capacitor is obtained by molding agglomerated particles of tantalum into a porous tantalum pellet and oxidizing the surface of the tantalum pellet to produce a dielectric oxide film. A tantalum electrolytic capacitor is then obtained by filling this anode with solid electrolyte and connecting a cathode to the impregnated solid electrolyte.
A process in which secondary particles of tantalum (secondary particles of tantalum resulting from agglomeration of primary particles, i.e. the smallest unit particles) are fabricated by tantalum salt reduction, e.g. using the regular melt reduction process, etc., and agglomerated particles of tantalum (tertiary particles of tantalum) are subsequently obtained by subjecting the secondary particles of tantalum to further agglomeration in a separate step has been proposed as a process for manufacturing agglomerated particles of tantalum intended for use in tantalum electrolytic capacitors. In order to address the miniaturization of capacitors that has occurred in recent years, there is a need for spheroidal particles with a small particle size that can be utilized as agglomerated tantalum particles for use in capacitors.
A process for obtaining agglomerated particles of tantalum from secondary particles of tantalum has been disclosed, for example, in Patent Document 1 wherein, after introducing water into a tantalum micropowder containing at least 50 wt % of particles with a grain size of 325 mesh or less, the powder is dehydrated to a moisture content of 2-30 wt %, dried in a stationary state, and heat treated in a vacuum.
Patent Document 2 discloses a process in which tantalum particles that have been pulverized to a median diameter (D50) of 50 μm or less in a granulating machine are wetted with a volatile liquid and granulated to form pre-granulated particles. Then, after drying in a stationary state, these pre-granulated particles are heat treated and classified by screening.
Patent Document 3 discloses a process in which tantalum particles are wetted and the resultant wetted particles are compacted and dried to form a cake and, after heat treatment, the cake is subjected to pulverizing, crushing, grinding, etc.
As mentioned earlier, in order to address the miniaturization of capacitors that has occurred in recent years, there is a need for spheroidal particles with a small particle size for use as agglomerated tantalum particles in capacitors. As far as primary tantalum particles are concerned, particles with a small particle size are required because they can increase the surface area of the tantalum pellet and can raise the electrical capacitance of the capacitor.
In addition, the agglomerated particles of tantalum have to be spheroidal particles with a narrow particle size distribution because they can increase the diameter of the voids in the tantalum pellet and improve the fill properties of the solid electrolyte.
Furthermore, the agglomerated particles of tantalum have to be particles with a low bulk density because they afford a high rate of compression during tantalum pellet molding and facilitate molding in predetermined shapes.
However, such low-bulk-density spheroidal agglomerated particles of tantalum with a small particle size and a narrow particle size distribution were not obtained in the manufacturing processes set forth in Patent Documents 1-3.
In addition, in the manufacturing processes for agglomerated particles of tantalum set forth in Patent Documents 1-3, attempts to reduce the mean particle size led to the formation of micro-particles with a size of several μm. For this reason, the particles had low flowability and could not be easily charged into the mold. In addition, the micro-particles generated in the course of manufacture or micro-particles generated subsequently by peeling, etc., used to get stuck in the gap between the female mold and the sides of the male mold, thereby making it impossible to extract the male mold from the female mold. Consequently, problems were likely to arise during pelletizing when sintered tantalum bodies were fabricated. Furthermore, due to the reduced porosity of the pellets made from the resultant agglomerated particles of tantalum, it was difficult to reduce the anode resistance of the anodes made from the pellets by impregnating them with a sufficient amount of solid electrolyte.
In this connection, the present applicants have proposed a process which involves, first of all, obtaining a granulated powder by combining a step of stirring while adding water with a subsequent step of stirring without adding water in a granulating apparatus, and then drying and sintering the powder (Patent Document 4).
In this process, micro-particles with a size of several μm are unlikely to be generated because of the high particle strength. For this reason, the powder has high flowability and can be easily charged into the mold. In addition, the problems associated with extracting the male mold from the female mold are also less likely to occur. Furthermore, pellets made from the resultant agglomerated particles of tantalum have sufficient porosity and, therefore, anodes can be easily impregnated with a sufficient amount of solid electrolyte and the anode resistance can easily be reduced.
However, upon further investigation, the present inventors found that the electrostatic capacitance per unit volume in pellets made from the agglomerated particles of tantalum obtained by this manufacturing process could sometimes be insufficient.